The assumption definitions are based on the measurements obtained in the second Six Sigma phase, described in Measure Phase. For example, Step 1, Customer Inquiry, has a mean of 1 hour and a standard deviation of 0.25 hour as discussed in Measure Phase. You can compare the other assumptions with the descriptions in Measure Phase to see how they match.
Notice that Step 3 is defined with a custom distribution. We will examine it in more detail shortly and then define the distribution in Step 5.